The present invention generally relates to a pressure relief valve controlled by a pilot valve, and more particularly, to an isolation system for isolating the pilot valve from process media or fluid.
A pressure relief system typically includes a relief valve mounted over a pressure vessel, such as a tank or pipe line, and a pilot valve which controls movement of the relief valve. The relief valve moves to an open position when pressure in the pressure vessel reaches a predetermined maximum level to discharge process media or fluid from the pressure vessel and reduce pressure therein. The relief valve moves to a closed position when enough process media is discharged to reduce the pressure in the pressure vessel to a desired predetermined level.
The pilot valve is typically of a non-flow type wherein process media does not flow through the pilot valve when the relief valve is in a static condition but process media does flow through the pilot valve to move or operate the relief valve. For example, see U.S. Pat. No. 3,664,362, disclosing a non-flow pilot valve, the disclosure of which is expressly incorporated herein by reference in its entirety. Process media flows through the pilot valve and changes a dome pressure of the relief valve to activate a pressure responsive element, such as a piston or diaphragm.
These pilot valves serve their intended purpose in an exemplary manner when the process media is clean. These pilot valves, however, are not suitable for dirty service wherein the process media is dirty and/or contains particulate or foreign matter because the pilot valves have many tight clearances which can be affected thereby. For example, flow passages can be clogged or restricted, sliding surfaces can be damaged and/or bound together, and sealing surfaces can be damaged and/or held apart.
One solution has been to supply an auxiliary source of clean fluid to the dome chamber of the relief valve and chambers of the pilot valve which expose a spool valve so that the process media does not contaminate the dome chamber or the spool valve. See, for example, U.S. Pat. No. 5,027,852, the disclosure of which is expressly incorporated herein in its entirety by reference. This solution, however, still exposes some components of the pilot valve to process media and cannot be used to retrofit existing pressure relief systems without replacing the existing pilot valve. Accordingly, there is a need in the art for an improved pressure relief system wherein a pilot valve is isolated from process media.